41CX current draw problem


I have a 41CX that eats its way through batteries way too fast, even when turned off. Anybody have suggestions what to look for when I open the unit up? BACKGROUND: I measured the current draw using a DMM and the calculator (without plugins) is drawing 0.54 mA from the batteries when turned off, compared to another (good) CX which drifted around between about .005 to .030 mA under the same conditions. OK, that explains the short battery life. But the bad calculator also draws 3.3 mA when turned on and idle, as opposed to 0.8 mA for the good one. While running CAT 0, 12.3 mA for the "bad" one, 8.3 MA for the good one. The higher draws when operating seem like a really bad sign. The calculator works fine, justs draws too much current. Suggestions what to look for?


An interesting feature of this problem is that the "excess" current increaces as the calculator demands more current.

To me, this suggests that there may be a problem in that part of the HP41 that provides a regulated rail.

Does the HP41 have a DC-DC converter?

I can't remember. But any problems with components that reduce efficiency may cause a problem similar to what you mention.

It could be a leaky transistor, a leaky cap... because of the high current in OFF mode, I'd not suspect resistors or inductors. Previous leakage of batteries may have damaged the PCB alowing leakage the of current that you witness.

Gee, "leak" is a useful word isn't it! (Leeks are also useful, but usually in soup...)


>Does the HP41 have a DC-DC converter?

Yes but not like that in the older calculators. It's almost entirely on one IC in the 41. HP calls it a "bi-polar supply IC". It generates 1.1, 2.2 & 3.3 VDC for the display and 2.6 and 6.3 VDC for the logic and some misc signals. Some of the voltages depend on the state that the 41 is in. The display voltages are temperature compensated so they will vary with temperature.

It's PN 1826--0566 in some models and 1826-0953 in others.



Has anyone worked out a partial schematic diagram for the processor daughter board inside the 41CX? I have traced my leakage problem up onto that board and need to understand the power routing on it. This is the smaller board, not the large board connected to the display. There are a dozen or so diodes, capacitors, etc on it in addition to the ICs. The calculator is SN 2417S42845, the processor board is PN 5081-5564 and is marked 408e28. I traced the + batt lead easily but would like to know which pad or trace is the -batt. Any help would be appreciated.



That current is way to high. You almost certainly have a bad power supply IC on the logic board. Send me the part number from the board and I'll tell you which IC it is. The only way to fix it is to replace the IC with one from another caclulator but I don't know if all of them use the same IC.



Joe - Thanks for the help, Your analysis sounds correct. I was badly underestimating how complex a power supply they would stuff into a hand held calculator. There is indeed a 14 pin DIP on the board marked 1826-0953-1. It is also marked RAYL 8350. Any idea if this could be a manufacturer's part number? As anyone looked? If not I could try and track it down once I get back to work after the holidays. Or could it just be a fancy HP date coding? The input leakage is now in the range of a few hundred ohms, but the display does try to come up if I force things with a power supply instead of a battery, so it is reasonable that only that converter chip is bad. Other numbers on the board: 5081-5564; 00041-60152 3 4 (with the 2 & 3 marked out using black ink); A-2216; hp408e28. Any ideas on finding a good board/broken calculator? Otherwise it becomes part of my spare parts collection, I guess.


Walt said "Other numbers on the board: 5081-5564; 00041-60152 3 4 (with the 2 & 3 marked out using black ink);"

Ok you have the board that HP calls the "HP 41 C/CV/CX Common Board". That was the last board produced for the non half nuts 41s. If only 41 C parts are installed then the 3 and 4 are marked out. If CV parts are installed the the 2 and 4 are marked out. If CX parts are installed then 2 and 3 are marked out. (That's what you have.)

The part is definitely a HP part. I'm not sure what the RAYL 8350 means but the 8350 sounds like a date code. Since you're running it off of a power supply with essentually unlimited power, you'll probably find that IC getting hot.

How handy are you with a soldering iron? There are a lot of HP 41s around that have battery terminals that have been eaten away by corroded batteries, or that have broken displays or other problems. You should be able to get one of them cheap. Any non-half nut model (C, CV or CX) will have the part that you need and that part seldom fails so even on a dead calculator there's a 99% chance that the part is good. If worse comes to worse you may want to buy a cheap working C just to get that part. A CX is worth more than a C.



Joe, Lots of very good information, thanks! You said "That was the last board produced for the non half nuts 41s." Does this mean I need to watch date codes in order to get the right board? Would the cutin date for the board be different for the C/CV/CX? Anyways, I will watch the ads and my local junk shops for a dead 41.



The half nuts were the later production HP 41s. They're recognizable from the outside by the rounded corners in the black shaded area around the edges of the display window. Check the museum webpage, I'm sure Dave has a picture of it shown. Internally everything is on one board (the keyboard). On the C and CV everything but the display is one IC! You can't go by the production date since the half nuts and standard model production overlapped for a year or more.


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